Spectroscopy systems and methods are used to analyze the composition of various types of materials, including gases, liquids and the like. Spectroscopy is based on the fact that different chemical compositions absorb energy, e.g., light, at different frequencies, so that measuring the intensity versus the frequency of the light passed through a sample can be used to identify which light frequencies were absorbed by the sample and which were not. Thus, the chemicals present in the sample can be readily identified. Spectroscopy systems and methods also can identify the amount of light absorbed by the sample at each given frequency. Thus, the quantity of each chemical present in the chemical composition can be determined. In addition, such analysis can be performed with any one of various different ranges of light such as infrared, ultraviolet and the like, each of which pertains to a separate range of frequencies.
An absorption cell (or resonator), which holds the gas or liquid sample through which light is passed, is used to perform spectroscopy analysis in conjunction with suitable spectroscopy equipment, such as a collimated laser light and a light detector. It has long been realized that to increase the sensitivity in providing both quantitative and qualitative analyses, the collimated laser light must be passed through a very large percentage of the available sample. Thus, absorption cells have been provided with “folded” light paths, in which mirrors reflect the light back and forth within the cell, such that the light makes multiple passes through the sample. The folded light path increases the optical path length between the laser and the light detector to thereby increase the sensitivity of a spectroscopy system incorporating an absorption cell producing a folded light path. Examples of existing “multi-pass” absorption cells are shown in U.S. Pat. Nos. 4,322,621; 4,749,276; 5,220,402; 5,440,143; 5,459,566; 5,485,276; 5,714,759; 5,731,583; 5,726,752; 5,818,578; and 5,949,537.
U.S. Patent Application Publication No. 2002/0185603 A1 ('603 publication), which is owned by the assignee of the present application, discloses an improved “multi-pass” absorption cell that increases the optical path length without greatly increasing the size or volume of the sample cell and causes light to pass through a very large percentage of a sample contained in the cell. The improved absorption cell of the '603 publication prevents the loss of significant amounts of light through ends of the cell and provides a large throughput, or etendue, so that the amount of light that can be directed through the sample and the sensitivity of the resulting measurement can be increased. In addition, the improved absorption cell of the '603 publication is robust and relatively easy to manufacture.
What is still desired is a new and improved multi-pass sample cell for use in absorption spectroscopy. Among other advantages and aspects, the new and improved multi-pass sample cell preferably will allow an absorption spectroscopy apparatus incorporating the cell to use light sources that are not collimated. In addition, the new and improved multi-pass sample cell preferably will minimize the difference (dispersion) in path lengths experienced by angularly divergent light rays traversing the sampling cell. The new and improved multi-pass sample cell preferably will also maintain or improve path length and throughput efficiency. Moreover, the new and improved multi-pass sample cell preferably will allow very high gas flow through the sampling cell while causing minimum disruption and turbulence to the gas flow. Furthermore, an absorption spectroscopy apparatus incorporating the new and improved multi-pass sample cell preferably will have a smaller total package size, including the source, the detector, the launch optics and the sample cell, in order to be fitted on existing flow streams.